1. Field of the Invention
The present invention relates generally to apparatus for performing surface analysis on a recording surface, and in particular, to an optimized piezoelectric contact detection sensor for detecting surface asperities on a magnetic recording disk.
2. Background
In a conventional magnetic storage drive, an air bearing slider supports a magnetic transducer in close proximity to a relatively moving recording surface. The recording surface typically comprises a rigid disk coated with a layer of magnetic material applied by a method such as spin-coating or sputtering. Coated disks must be free of asperities to assure long-term reliability and the data integrity at the head to disk interface, since asperities can lead to undesirable slider-disk contact or "head crash".
Glide height testing is one means for assuring an asperity-free disk. A slider is flown over the recording disk at a height equal to or below the desired data head fly height to analyze impacts between the slider and the disk surface. The slider includes one or more piezoelectric sensors bonded to an upper surface facing away from the recording surface. Piezoelectric materials are used because they generate an electric charge in response to internal stress. As the slider experiences rigid body displacement and flexural deformation, the adjacent sensor responds by generating a charge signal which may be monitored.
A dominant practice in the art has been to monitor the low frequency piezoelectric signals corresponding to rigid body displacement and indicative of slider contact with large asperities on the disk surface. But as sliders decrease in size, magnetic transducers become vulnerable to relatively small asperities. Experience among those skilled in the art has shown a class of asperities (e.g. disk delaminations) that are too small to cause head crashes, yet large enough to result in slider-disk contact adversely affecting device reliability. This class of asperities generates high frequency vibrations in the test slider which cannot be detected adequately by conventional means.
The optimal sensitivity to small disk asperities is obtained by monitoring the high frequency vibrations of a test slider. Yet the high frequency components, or bending mode frequencies, of the response signal may vary greatly. Many modes display a non-monotonic response with increasing asperity interference height, i.e. the distance between the tip of an asperity and the minimum slider fly height. Nonmonotonic modes indicate the occurrence of disk contact but provide no useful information about the size of the asperity causing contact.
The trend in recent years has been to produce storage systems having smaller sliders than the conventional "large" or "100%" sliders (e.g. 4 mm long.times.3.2 mm wide). Reductions in slider size necessitate a corresponding reduction in test slider dimensions for equivalent compliance to the recording surface. This reduction results in a weaker piezoelectric signal and poor signal-to-noise (S/N) ratio. S/N ratio has also been shown to decrease with decreasing glide height. Thus, optimizing test slider sensitivity becomes increasingly important for smaller slider designs.
U.S. patent application Ser. No. 08/174,484, filed concurrently herewith, discloses a method and apparatus for identifying the high frequency bending mode components of a signal response generated in a test slider during small asperity contact. The application also discloses a method and apparatus for analyzing the identified components to select an optimal monotonic bending mode for increasing asperity interference height, and a method for designing a piezoelectric sensor optimized to detect the identified mode. What is needed is a piezoelectric sensor having optimized sensitivity to a predetermined high frequency bending mode signal generated in a small slider during contact with small surface asperities, and a method for making the same.